State-of-the-art pacemakers, ICDs and other cardiac rhythm management devices (CRMDs) can be equipped with radio-frequency (RF) communication devices for communicating with external systems such as bedside monitors or external diagnostics systems. In particular, RF communication devices have been developed to utilize Medical Implant Communication Service (MICS)-band radio transmissions or Medical Device Radiocommunications Service (MedRadio)-band transmissions. (MedRadio maintains the spectrum previously allocated for MICS (402-405 MHz) while adding additional adjacent spectrum (401-402 MHz and 405-406 MHz).) Herein, the term “MICS/MedRadio” will be used for the sake of completeness and generality to refer to MICS, MedRadio or both.) RF capable devices use an antenna within the header or adjacent header for receiving or transmitting RF signals. However, problems arise in designing such antennas due to the increasing miniaturization of CRMDs and their components.
In particular, there can be a loss of RF communication performance due to the reduction in size of the header and the device case (also called the housing or the “can”) of the CRMD. As technology improves, the sizes of the implantable devices continue to shrink but the laws of physics regarding RF communications do not change. Since about 2005, at least some CRMD designers have employed a shorted loop antenna for RF communications. However, RF computer simulations indicate that a further reduction in device size would diminish antenna performance below acceptable levels. Accordingly, there is a need to provide improved antenna designs for use with CRMDs, especially relatively small devices.
In this regard, there are many challenges to designing a well performing antenna for use within an implantable medical device. One issue is the significant amount of attenuation inherent to the system since the RF signal travels through the lossy human body. Another problem is that the size of the antenna is limited by the size of the header (at least for devices where the antenna is to be fitted inside the header.) Ideally, the antenna should have a length equal to a quarter wavelength of the operating frequency (which is typically near 400 MHz), but it is difficult to design an antenna that fits within a device header while achieving that length. Hence, for antennas to be housed in the device header, the quarter wavelength constraint can result in an antenna much smaller than needed for optimum performance. Another issue is that the antenna should have an input impedance that is the complex conjugate of impedance of the internal circuitry of the device so maximum power transfer can take place. If the impedance of the antenna is too low or too high, additional mismatch losses will occur, which will decrease signal power.
FIG. 1 illustrates an antenna 2 that attempts to meet these requirements using a folded monopole design commonly known as an “Inverted L antenna” for use within the header 4 of an exemplary CRMD 6. The Inverted L is a monopole that ideally should be sized to a quarter wavelength of its operating frequency with a 90-degree bend to resemble a downward facing L. The antenna can fit within a fairly small header volume but suffers from very low input impedance. Also, this antenna is best suited for higher gigahertz (GHz) frequency applications where the necessary antenna length for resonance is relatively short. At 400 MHz, implementing an Inverted L antenna becomes impractical for implantable device purposes, as this would require a very long antenna that would not fit within the header. To solve the impedance issue, an extra branch 7 can be connected to the Inverted L and shunted to ground. This topology, shown in FIG. 2, is known as the “Inverted F antenna.” (An Inverted F antenna design is discussed, for example, in U.S. Pat. No. 7,047,076 to Li et al., entitled “Inverted-F Antenna Configuration for an Implantable Medical Device.”) The extra shunt connection provides a larger input impedance for matching purposes but the Inverted F still suffers from lack of adequate length for practical applications wherein the antenna must fit within the header of a relatively small CRMD.
Accordingly, there is a need to provide an improved antenna, particularly for MICS/MedRadio applications, that addresses these and other issues. It is to this end that aspects of the invention are generally directed.